Field of the Invention
The presently disclosed invention relates to an apparatus for making low carbon steel and, in particular, improved snorkels for conveying molten metal between the ladle and a vacuum vessel.
Discussion of the Prior Art
For many years it has been known that workability of steel can be significantly improved by decreasing the carbon content of the steel. More recently, there has been a growing demand for low carbon steel. In some applications such as thin gauge steel that is used in automotive applications, it is preferred to use ultra low carbon steel in which the carbon content is reduced to about 0.005%.
In the process for making ultra low carbon steel known as the RH process, the carbon content of the steel is reduced by lowering the partial pressure of carbon monoxide at the surface of the molten metal. More specifically, the molten metal is drawn from the steel ladle into a vacuum vessel that is located above the ladle. It is known in the art to locate two snorkels at ports in the bottom of the vacuum vessel and that extend downwardly toward the steel ladle. The snorkels are sufficiently long that when the vacuum vessel and the steel ladle are brought vertically closer together, the free ends of the snorkels extend into the steel ladle to an elevation below the normal surface of the molten metal.
One of the snorkels designated as the “up leg snorkel” incorporates passageways for an inert gas such as argon. At times when the free end of the up leg snorkel is below the surface of the molten metal in the ladle and a partial vacuum is established in the vacuum vessel, inert gas is injected into the molten steel inside the up leg snorkel to support the upward movement of the molten steel through the up leg snorkel and into the vacuum vessel. This also creates turbulence in the molten metal to increase the efficiency of the process by increasing the rate of carbon removal. Molten metal in the vacuum vessel then re-enters the steel ladle through the “down leg” snorkel.
Processing time for circulation of the molten metal through the vacuum vessel is typically about thirty minutes. During that time, the snorkels are exposed to the molten metal so that the temperature of the snorkels significantly increases. Molten metal is located both inside and outside the snorkels so that heat from the molten metal penetrates the snorkels both from the inner bore and from the outer surface.
Typically, the snorkels are constructed of a steel shell with the surface of the inner bore and the outer surface of the snorkel protected by refractory materials. The coefficient of thermal expansion of the steel shell is greater than the coefficient of thermal expansion of the refractory materials. Therefore, prolonged heating of the snorkel has resulted in cracks in the outer layer of refractory concrete. The refractory cracks allow subsequent penetration of the molten steel. Unless the snorkel is taken out of service and the refractory concrete repaired or replaced, the cracks will ultimately lead to catastrophic failure of the snorkel.
Similarly, the inner refractory material is a brick layer. The brick layer is steadily eroded by the turbulent action of the molten metal caused by the injection of the inert gas. As the brick layer grows thinner, the rate of heat transference from the molten metal to the steel shell increases. Again, unless the snorkel is taken out of service and the brick layer repaired or replaced, the brick layer will present an insufficient thermal barrier and lead to catastrophic failure of the snorkel. Accordingly, it was recognized in the prior art that systems or methods for retarding the rate of heating of the steel shell in the snorkels would advantageously increase the number of heats in which a snorkel could be used without taking it out of service for repairs.
In some prior art snorkels, an array of pipes has been secured to the surface of the steel shell. The pipes are used to convey a cooling medium such as air to and around the steel cylinder to retard temperature increases of the steel cylinder during times that the snorkel is exposed to the molten metal. This arrangement has had some success, but its capability is limited in certain important respects. One significant limitation has been that the cooling capacity is proportional to the volume of cooling medium that is exposed to the steel cylinder. In the prior art, the volume of cooling medium is limited by the size of the pipes in the piping array. The size of the pipes used for conveying cooling medium, and thus the cooling capacity, is limited by the physical geometries of the snorkel.
An example of such prior art snorkels that is shown and described in JP Publication 2004256881 includes inner and outer concentric tubes with refractory materials on the inside of the inner tube and on the outside of the outer tube. Cooling gas is delivered through ports in the outer tube to a space between the concentric tubes. Fins that are oriented parallel to the longitudinal axis of the tubes are secured to surfaces of the inner and outer tubes that define the concentric space therebetween. The fins convey heat from the inner and outer tubes and thereby enlarge the effective area for dissipating heat to the cooling gas that flows through the space between the tubes.
JP Publication 11080828 shows another prior art snorkel in which concentric double tubes include a helical-shaped pipe in the space between the tubes. The flow of cooling gas between the double tubes is augmented by cooling water that is pumped through the pipe to transfer heat away from the tubes. Alternatively, the space between the tubes can include plates that are oriented generally parallel with the longitudinal axis of the tubes and arranged in an alternating fashion such that the plates form a pattern of openings that alternate between the top and bottom ends of the space. The pattern of openings results in a vertically undulating flow of cooling air through the space.
Such prior art designs tended to oppose the vertical thermal convection of the heated air. Also, these designs allowed only a unitary flow path for the cooling air throughout the interstitial space between the concentric tubes.
Also, improved safety features in degasser snorkels would be desirable. For example, some prior art designs proposed the use of water as a cooling medium. While using water may offer certain advantages in terms of thermal transfer capabilities, it also creates a severe explosion hazard in the event that the water should escape the cooling system and be directly exposed to the molten steel. Furthermore, water-based systems had inherent limitations in that, among other reasons, they were typically designed to work at relatively low pressures to maintain laminar flow through the system. Although higher pressures would have improved thermal transfer capability, it was found that higher pressures caused turbulence in the flow of the cooling water and resulted in dead spots in the flow path. Such dead spots were undesirable in that they caused the accretion of particulates and precipitates that tended to obstruct the flow path.
Systems that used an air cooling medium avoided the explosion risks of liquid systems, but are less efficient in transferring heat out of the snorkel. To improve efficiency, air-based systems sometimes proposed higher pressures that would create turbulence in the air flow. The turbulent air flow through the cooling passages would better convey heat, but the higher pressures created a high risk that air would escape the cooling passageways of the snorkel and become exposed directly to the molten steel so as to cause an explosion. Accordingly, air systems that operated at lower pressures would be preferred. However, to maintain adequate thermal capacity for the snorkel, the lower pressure air systems typically required larger passageways. In many cases, the snorkel became too bulky or structurally unsuited to accommodate adequate passageway geometries. Accordingly, there was a need in the prior art for a snorkel that had a cooling capability for an air medium that would operate are relatively low pressure.
Accordingly, there was a need in the prior art for an apparatus that could more effectively cool the steel cylinder of the snorkel without otherwise compromising the performance of the apparatus and method for making ultra low carbon steel.